In the past, different methods have been used for welding sheet-like materials together, for example traditional heat contact welding. In this method the two sheets to be welded together are placed or clamped between a heated welding element and a support element such that the sheets are pressed against each other with a given force to ensure close contact between the surfaces to be welded. Heat is then applied from one side through one of the sheets until, by heat transfer, it reaches the second sheet; typically materials are chosen such that both materials will melt and a weld will be established between the two sheets. This method necessitates free access to both sides of the two-layer assembly.
A different method used is HF welding in which the two sheets to be welded together are placed or clamped between a support element and an element from which a radio frequency signal is directed into the layers, this resulting in “internal” heating of the materials. Due to radiation from the surfaces of the materials to be welded, heat build-up is strongest towards the centre of the two-layer assembly which, when the layers have the same thickness, is also the location for the area to be welded, i.e. the contacting surfaces. Drawbacks of this method are that the materials must be sensitive to HF, e.g. PVC. The materials and the HF process are generally environmentally hazardous, and the difficulties associated with welding materials with different thickness limit the applications.
A further method is laser welding using a CO2 laser which tends to heat most plastics from the surface down with a very rapid heating action achievable; in this way it resembles traditional heat transfer welding. Nd:YAG and diode laser light will transmit through un-pigmented polymers. The polymer can be designed to absorb and heat in these laser beams due to inherent properties or with the addition of an absorber. This process was first described in 1985 for welding automotive components; see Japanese patent application JP85-213304 “Laser beam welding of plastic plates” published Sep. 26, 1985 and known as transmission laser welding. Typically carbon black pigment is used as the additive to enhance absorption from transmission laser welding, however, carbon black, which results in a black material when added, is traditionally considered unsuitable for medical use in which a transparent appearance is in most cases desirable. It is also possible to use materials which are opaque instead of black; however, they still lack the desirable fully transparent appearance.
The above-described principle is also known from EP-A-0 942 139 which discloses a method for laser welding transparent window panes to a frame, the frame at least at the abutment surfaces absorbing the laser light, this making it necessary to provide the laser light from each side of the frame.
However, for a number of different products it is necessary to establish a weld between only two layers within a product which in its finished state is a multi-layer assembly.
A first example of this kind of product is ostomy pouches that ordinarily consist of two pouch walls welded together along their contour. One of the walls has an aperture which receives material discharged from the stoma. A coupling element, e.g. an adhesive label or a faceplate, is affixed to the pouch wall around the stomal aperture by welding or adhesion. The coupling element permits the detachable fastening of the pouch on the user's body around stoma.
In the production of such ostomy pouches, the starting material used is generally two webs of weldable plastic sheet material that are advanced stepwise through an apparatus with three stations. The first station has a punching tool to produce the apertures in one of the webs, the coupling element being welded onto said web around the apertures at the second station, and the two webs being subsequently brought together and united along a line corresponding to the desired contour of the pouch by means of an annular welding electrode at the third station. The finished pouches may simultaneously be separated from the remaining sheet material.
As the contour welding operation cannot be carried out through the coupling element it is a primary condition in this technique of production that the coupling element be positioned entirely within the contour of the pouch. In many cases, this is completely acceptable. However, in other cases, it means that for production reasons the pouch must be made bigger than otherwise necessary or that the distance from the aperture to the top of the pouch must be increased. The use of a pouch having such increased dimensions can result in the upper part of the pouch pulling away from the user's body as a consequence of the increase in weight when the pouch fills.
According to EP-A-0 045 587, a method of manufacturing ostomy pouches in which an adhesive label or faceplate can extend past part of the pouch contour is suggested. The particular feature of this method consists in that the two pouch walls in a first step are joined by partial contour welding, namely over an area extending from the top edge of the pouch past the region of the weld seam around the aperture. The adhesive label or faceplate is subsequently applied by welding while using a separator member introduced into the partially finished pouch. The separator functions as a base for the pouch wall to which the label is being welded. The contour welding operation is subsequently completed in a second step after withdrawal of the separator member. Although EP-A-0 045 587 discloses a method which is suitable for the manufacture of the desired ostomy pouch, this method is relatively complex using a large number of manufacturing steps as well as special equipment and is thus cost-ineffective.
In the above mentioned example, placement problems of a coupling element such as an adhesive label are discussed, but the same considerations are valid in other cases, e.g. when external components such as filter assemblies with similar dimensional features are to be fixed on a pouch.
Another type of product featuring a multi-layer construction with welds between only two layers within the finished product is ostomy and urine collecting bags with internal valves formed between one or two additional internal sheet-elements.
An example of such a product is known from EP-A-0 106 587 which discloses a drainage bag which incorporates a non-return valve assembly. More particularly, the valve assembly includes at least one baffle positioned intermediate the top and bottom of the bag and extending across the bag from its left side to its right side. Each baffle has an upper portion which is continuously attached to one face of the bag from the left side of the bag to the right side of the bag and a lower portion which is attached to an opposite face of the bag at spaced intervals between the left side of the bag and the right side of the bag. By this construction, at least one opening is formed along the bottom portion of the baffle, the opening or openings being positioned adjacent to the front or back face of the bag so as to permit liquid to flow readily from the top of the bag to the bottom of the bag while inhibiting liquid from flowing from the bottom of the bag to the top of the bag. Although EP-A-0 106 587 discloses the use of a single baffle member, non-return valves can also be formed with two intermediate baffle layers, each attached to a wall of the pouch.
A further type of product featuring a multi-layer construction with welds between only two layers within the finished product is ostomy and urine collecting bags with internal walls attached between the outer front and back walls in order to reduce bulking when the bag is filled. Such a bag as well as methods for the manufacture thereof is disclosed in WO 93/17643.
As follows from the description of the just described two types of bags, such bags comprise at least three layers making up the outer front and back walls and the baffle member/internal wall of the bag, respectively, the inner layer being attached to the outer walls along welding seams.
U.S. Pat. No. 5,165,799 discloses a method in which a curable adhesive is applied, e.g. printed, to selected surface portions of layers in a multi-layer plastic bag. The adhesive is cured by an electron beam curtain emitted across the full width of the bag. Although this method allows for selective bonding of individual portions of layers in a multi-layer construction, the accuracy relies entirely of the positioning of the printed layers prior to curing, this making the method unsuitable for products in which a high degree of precision is required, such as for medical appliances. Further, for some medical appliances, a welded bond may be required.
As appears from the above, it would be desirable if multi-layer assemblies of the described types could be manufactured in a simple, reliable and cost-effective way providing a high degree of precision. Therefore, it is the object of the present invention to provide a method which makes it possible to manufacture an assembly in which a weld is established between only two layers at a given location within a product which in its finished state is a multi-layer assembly, and which circumvents one or more of the above-described drawbacks of the prior art.